Regional Groundwater Model of the Regional Municipality of Durham

Regional Groundwater Model of the Regional Municipality of Durham

Client: Conservation Authorities of the Moraine Coalition, 5 Shoreham Drive, Downsview, Ontario. Steve Holysh, Chief Hydrogeologist, 416-661-6600 ext. 5588

Key Personnel: E.J. Wexler, D. Kassenaar, A. Kodippili, John Ford.

The Regional Municipality of Durham lies east of Toronto and encompasses an area of approximately 2,600 square kilometres. Population centres within the area include a number of urban communities along the Lake Ontario shoreline and a variety of small towns, villages, and hamlets that lie further inland. The region includes portions of the Lake Simcoe and Lake Scugog shorelines. The Oak Ridges Moraine is a significant physiographic feature and an area of high groundwater recharge and runs parallel to the shoreline about 24 kilometres north of Lake Ontario.
Municipal water supply for the larger towns is taken directly from Lake Ontario, however, most of the inland towns rely on municipal groundwater supply. Durham Region operates 27 municipal wells in eight communities. Approximately 18,350 private water wells have also been drilled. Population has risen sharply in the past decade and continued growth is expected. Protection and management of the groundwater resource is needed to sustain development.

To advance the understanding of the groundwater resources, a 10-layer numerical model was developed to simulate groundwater flow in Durham Region. The “Durham Model” extends to natural hydrologic boundaries (i.e., Lake Ontario, Lake Scugog, Lake Simcoe, major groundwater divides, and rivers centrelines). Model layers represent the major hydrostratigraphic units which reflect the geology in the Quaternary-aged unconsolidated sediment and the shallow bedrock. A key improvement over earlier work is that the “Newmarket Till” has been subdivided into three units: an Upper and Lower Newmarket Till Aquitard and an intervening Inter-Newmarket Sediment (INS) Aquifer.

A detailed water budget was conducted for the southern part of Durham Region using the USGS Precipitation-Runoff Modelling System (PRMS). The PRMS model takes information of daily climate, soil properties, land use, and topography to derive estimates of overland runoff, groundwater recharge and baseflow. The model was calibrated to observed streamflow at Environment Canada gauges. Rates of annual average groundwater flow from the PRMS model were applied to south Durham and were used to improve recharge estimates for the north.
The groundwater model was calibrated to match observed water level data (MOE WWIS data along with other geotechnical and consultant wells, and average water levels from long-term monitoring sites). Another target for model calibration was estimates of annual average baseflow at the EC gauges. The calibrated model produced a detailed groundwater budget, simulated heads and flow rates in each model layer, and estimates of groundwater discharge to all streams in the study area.

The numerical model was provided to CAMC-YPDT and Durham Region at the end of the study for use in Source Water Protection studies, engineering investigations, wellfield protection studies, wellfield optimization, and monitoring network design.